The present invention relates to improved trivalent chromium electrolytes, and more particularly to an improved trivalent chromium electrolyte composition which is substantially more tolerant to the presence of deleterious contaminating metal ions such as nickel, zinc, iron, copper and lead as well as organic contaminants such as excess organic wetting agents or brighteners, which accumulate and progressively increase in concentration during normal commercial operation of the plating bath. When one or more of such metal ion impurities attain concentration levels in which they are present in even relatively trace quantities, the chromium electrodeposit is adversely affected by the presence of black streaks, clouds and hazes which are sometimes further accompanied by a loss or reduction in covering power rendering such chromium electrodeposits commercially unsatisfactory. Similarly, excessive organic contaminants can give rise to the presence of white streaks thereby flawing the resulting deposit.
In recognition of the problem associated with extraneous metal ions and organic contamination of such trivalent chromium plating baths, it has heretofore been proposed in accordance with U.S. Pat. No. 4,038,160 to add small amounts of water soluble ferrocyanide compounds to the plating solution to effect a precipitation of such contaminating metal ions which thereafter are removed by filtration. While such proposed ferrocyanide treatment has been found effective in many instances, the treatment is costly and time consuming and the ferrocyanide precipitating agent itself can adversely affect the performance of the trivalent chromium electrolyte when employed in amounts that leave a residual excess of the precipitating agent dissolved in the bath. This necessitates further treatment by the intentional addition of contaminating metals to effect a precipitation of the excess precipitating agent.
It has also been proposed to remove such contaminating metal ions through an electrolytic purification and peroxide treatment technique by which the bath is electrolyzed over a period of time employing a cathode on which a codeposition of the contaminating metal ions is effected. Unfortunately, while such an electrolytic purification technique is somewhat effective for reducing copper ion contamination, it is relatively ineffective for removing nickel and zinc ions and is only partially effective for removing iron.
Other techniques have been proposed for the removal of extraneous metal ions and organic contamination. For example, U.S. Pat. No. 4,432,843 suggests the use of thiazole and benzothiazole compounds. This treatment is relatively costly and time consuming and therefore has not proven successful.
The present invention provides an improvement over such prior art techniques by providing a trivalent chromium electrolyte which is more tolerant to the presence of one or more of such contaminating metal ions or organic contaminants masking or hiding their deleterious effects thereby providing for a longer useful operating life of the bath under normal commercial operating conditions. Additionally, the present invention enhances the codeposition of such contaminating metal ions thereby substantially reducing the rate of buildup of the concentration of such contaminating ions during the normal commercial operation of the bath. In those instances in which the rate of contamination is relatively low, such enhanced codeposition is adequate in and of itself to prevent accumulation of such metal ions to levels at which deleterious results are obtained. The present invention further contemplates a method for rejuvenating or restoring the performance of a trivalent chromium electrolyte which has been detrimentally affected by the accumulation of such contaminating metal ions whereby the concentration thereof is reduced restoring the electrolyte to commercially satisfactory operating conditions.
Additionally, the present invention enhances the deposit's ability to be color buffed after plating. Prior trivalent chromium electrolyte formulations as described, for example, in U.S. Pat. No. 4,473,448 result in chromium electrodeposits which exhibit poor color buffing properties. A 0.3 mil nickel plated brass Hull cell test panelplated for three minutes at 5 amps with the electrolyte of the '448 patent was color buffed using an 8282 cloth wheel and Lea Rok scratchless pink coloring composition. Upon examination, the panel exhibited areas wherein the chromium deposit was removed revealing the nickel underplate. When the additive of the present invention was added in the amount of 30 g/l to the electrolyte of the '448 patent (Ex. 1 with thiourea) and a 0.3 mil nickel plated brass Hull cell test panel was plated for 3 minutes at 5 amps, the resulting panel was color buffed in the same manner as described above. Upon examination, the panel exhibited no areas of cut through and the chromium deposit remained intact. It is important to note that while both examples provided commercially acceptable deposits, the ability to color buff afforded by the additive of the present invention indicates a harder and/or thicker deposit is obtained.